For example, in order to improve gas turbine (G/T) combined cycle efficiency, a heat recovery steam generator for effectively utilizing combustion flue gas from a gas turbine is used. This heat recovery steam generator (HRSG) is an apparatus that generates steam using a high-temperature combustion flue gas discharged from an exhaust heat generation source, such as a gas turbine, and is widely used in, for example, a gas turbine combined cycle (GTCC) power generation plant that supplies steam generated in the heat recovery steam generator to a steam turbine (S/T) and drives a power generator (PTLs 1 and 2).